This invention relates to electrodes used in transcutaneous cardiac defibrillation and pacing procedures.
Transcutaneous cardiac defibrillation is an emergency procedure for treating ventricular fibrillation, a condition in which the electrical pulse generators in the cardiac muscle fibrillate asynchronously, causing chaotic muscle contraction. In the procedure, a high energy electrical pulse, called a defibrillation pulse, is transcutaneously delivered to a patient's fibrillating heart to resynchronize the heart's pulse generators. In transcutaneous cardiac pacing, pacing stimuli are transcutaneously delivered to a patient's heart to continuously pace the heart.
Defibrillation pulses and pacing stimuli are transcutaneously delivered from pulse generation equipment to a patient via a pair of electrodes applied to the patient's thorax in a suitable configuration. Typically, either of two types of electrodes is used; the first type comprises separate, dedicated defibrillation and pacing electrode pairs, while the second comprises a multifunction electrode pair which supports both defibrillation and pacing procedures.
The multifunction electrode typically consists of a flexible adhesive substrate, supporting a conducting plate, which is temporarily affixed to the patient's skin, and so does not require an operator to forcibly hold it in place on the skin. This electrode is designed to be used for one treatment session and then discarded.
It is desirable to provide a uniform contact surface between the multifunction electrode conducting plate and the patient's skin. To this end, a water-based electrolytic gel is typically provided on the electrode conducting surface. With the electrode in place on the patient's thorax, this gel soaks the skin, allowing the electrolytes in the gel to permeate the skin and thereby provide a good conducting path for the defibrillation and pacing stimuli. In addition, the gel wets hair on the patient's skin and provides a good conductive path around the hair and into the skin. The electrodes are typically gelled during the manufacturing process and require no further preparation before use.
Conventionally, the components of the aqueous electrolytic gel are chosen to achieve very low gel resistance, and thus very high gel conductivity, to minimize the pulse energy dissipated in the gel and thereby maximize the defibrillation pulse energy and pacing stimulus delivered to the patient. The electrical resistance of a patient's thorax is believed to range somewhere between 25 .OMEGA. and 100 .OMEGA., and is typically modelled as 50 .OMEGA.; the series resistance of the pair of multifunction electrodes, including electrode gel, is held below 1.0 .OMEGA..
In defibrillation procedures, typically more than one defibrillation pulse is required to successfully defibrillate a patient's heart. Being affixed to the patient's skin at the start of a defibrillation session, multifunction electrodes do not change position with each pulse application. It has been clinically observed that with repeated defibrillation pulse applications, some burning of a patient's skin may occur at the perimeter of the gel layer of each of the multifunction electrodes. This burning is characterized by erythema across a thin band at the gel perimeter location. It is believed that the location of the burn is determined by the spatial distribution of the defibrillation pulse current across the electrode and gel face; this current is highest at the perimeter of the gel, due to the abrupt boundary of the electric field at this perimeter. The electrode burn is exacerbated by repeated defibrillation pulses because the multifunction electrodes are maintained in a fixed position throughout a pulse series.
In conventional transcutaneous pacing procedures, the patient may experience a stinging of the skin in the area of the electrodes' positions. This stinging is believed to also be related to the high current level of delivered stimuli at the edge of the conducting plate and gel.